SEX AND SEX DIFFERENCES AS QUANTITATIVE

Intersexes in moths; Bird intersexes; Higher metabolism of males; Quantitative difference between sex factors; Old ideas of intersexuality; Modern surgery and human intersexes; Quantitative theory a Mendelian explanation; Peculiar complication in the case of man; Chemical life cycles of the sexes; Functional-reproductive period and the sex problem; Relative significance of physiological sex differences.

Crossing European and Japanese gypsy moths, Goldschmidt [1,2,3,4] noticed that the sex types secured were not pure i.e., that certain crosses produced females which bore a distinctly greater resemblance to the male type than others, and vice versa. One of these hybrids of “intersexes,” as he calls them, would always possess some female and some male sexual characters. He found that he could separate the males and females, respectively, into seven distinct grades with respect to their modification toward the opposite-sex type, and could produce any one of these grades at will by breeding.

For example, the seven grades of females were roughly as follows: (1) Pure females; (2) Females with feathered antennæ like males and producing fewer than the normal number of eggs; (3) Appearance of the brown (male) patches on the white female wings; ripe eggs in abdomen, but only hairs in the egg-sponge laid; instincts still female; (4) Instincts less female; whole sections of wings with male colouration, interspersed with cuneiform female sectors; abdomen smaller, males less attracted; reproduction impossible; (5) Male colouration over almost the entire wing; abdomen almost male, with few ripe eggs; instincts intermediate between male and female; (6) Like males, but with rudimentary ovaries and show female traits in some other organs; (7) Males with a few traces of female origin, notably wing-shape.

The males showed the same graded approach to the female type. Their instincts likewise became more and more female as the type was modified in that direction. That is, a moth would be 12% or 35% female, and so on.

Goldschmidt watched the crosses which produced seven different grades of maleness in his females. The moth material, like the birds and mammals, suggested a dual basis for sex in each individual. The grades of maleness and femaleness made it seem probable that the factor which determines sex must be stronger in some instances than in others, i.e., that the difference between two of these grades of female is originally quantitative, not qualitative in amount rather than in kind.

Mating European moths with European, or Japanese with Japanese, produced pure, uniform sex-types, male and female. But a cross of European with Japanese strains resulted in intersexes. Goldschmidt concluded that (1) all individuals carried the genetic basis for both sexes; and (2) that these basic factors were two chemicals of enzyme nature. One of these he called Andrase, enzyme producing maleness, the other Gynase, enzyme producing femaleness. Further, (3) since each chemical sex determiner is present in both individuals in every cross, there must be two chemical “doses” of maleness and two of femaleness struggling for mastery in each fertilized egg. (4) If the total dose of maleness exceeds the total dose of femaleness, the sex will be male, and vice versa. (5) These quantities get fixed by natural selection in a single race which always lives in the same environment, i.e., the doses of maleness and femaleness in a given sex always bear practically the same relation to each other. Hence the types are fixed and uniform. (6) But different races are likely to have a different strength of chemical sex-doses, so that when they are crossed, the ratios of maleness to femaleness are upset. Often they are almost exactly equal, which produces a type half male and half female or 2/3, or 1/3, etc. The proof of this theory is that it solved the problems. Goldschmidt was able to work out the strengths of the doses of each sex in his various individuals, and thereby to predict the exact grade of intersexuality which would result from a given cross.

Riddle’s work on pigeons [5,6] brings us much nearer to man, and suggests the results noted by both Goldschmidt and Lillie. As in the Free-Martin cattle, there is an apparent reversal of the sex predisposition of the fertilized egg. As in the gypsy moths, different grades of intersexes were observed. In the pigeons, it was found that more yolk material tended to produce a larger proportion of females. The most minute quantitative measurements were made of this factor, to eliminate any possibility of error.

The chromosome mechanisms practically force us to suppose that about half the eggs are originally predisposed to maleness, half to femaleness. A pigeon’s clutch normally consists of two eggs, one with a large yolk and one with a small yolk. But the half-and-half numerical relation of males to females varies considerably i.e., not all the large-yolked eggs produce females or all the small-yolked ones males.

Wild pigeons begin the season by throwing a predominance of males, and the first eggs of the clutches also tend to produce males all along. In both cases, the male-producing eggs were found to be the ones with the smaller yolks. Family crosses also produce small yolks, which hatch out nearly all males. Some pairs of birds, however, have nearly all female offspring. Riddle investigated a large number of these cases and found the amount of yolk material to be large. In other words, there seems to be a definite relation between the amount of yolk and sex.

A great number of clever experiments were carried out to find out if eggs originally predisposed to one sex were actually used to produce the other. Selective fertilization with different kinds of sperm was impossible, since in these birds there is only one type of sperm two of eggs as to the sex chromosome. For instance, by overworking females at egg-production, the same birds which had been producing more males than females were made to reverse that relation.

One of the interesting results of the experiments was the production of a number of intersexual types of various grades. This was easily verifiable by colour and other characteristics. To make sure that the instincts were correspondingly modified, behaviour was registered on moving-picture films. Where the first egg of a clutch (the one with a small, normally male-producing yolk) produces a female, she is usually found to be more masculine than her sister from the second egg with the larger yolk. This is true both as to appearance and as to behaviour. Some of these were quite nearly males in appearance and behaviour, though they laid eggs.

Testicular and ovarian extracts were injected. The more feminine birds were often killed by the testicular extract, the more masculine by the ovarian extract. Finally, to make assurance doubly sure, some females which should theoretically have been the most feminine were dissected and shown to be so. That is, out-crosses which produced a predominance of females in the fall were mated with females which had been overworked at egg production until they threw nearly all females. Dissecting the females thus produced, they were shown to have right ovaries, which means double femaleness, since normally the pigeon is functional only in the left ovary, like other birds. The right one usually degenerates before or at hatching and is wholly absent in the week-old squab.

In pigeons, Riddle thinks the “developmental energy” of the eggs is in an inverse ratio to their size. The last and largest eggs of the season develop least and produce most females. The second egg of a clutch is larger than the first, but develops less and the bird produced is shorter-lived. Overworking and other conditions tending to produce large eggs and females also throw white mutants and show other signs of weakness. Old females lay larger eggs than do young ones. These eggs produce more females. They store more material, have a lower metabolism and less oxidizing capacity than do the earlier male-producing eggs.

It would be unsafe to draw specific conclusions about mammals from these bird and insect experiments. Both the secretory action and the chromosome mechanisms are different. The quantitative nature of sex, and also the existence of intersexual types, between males and females, would seem to be general phenomena, requiring rather slight corroboration from the mammals themselves. We have such mammalian cases as the Free-Martin cattle, and some convincing evidence of intersexuality in the human species itself, which will be reviewed presently.

The notion of more “developmental energy” or a higher metabolism in males is borne out in the human species. Benedict and Emmes have shown by very careful measurements that the basal metabolism of men is about 6% higher than that of women. Riddle cites the work of Thury and Russell on cattle to show that a higher water value (as he found in the pigeon eggs), associated with increased metabolism, helps to produce males.

In males, the secretion of the sex glands alone seems to be of particular importance, again suggesting this idea of “strength” which comes up over and over again. Removal of these glands modifies the male body much more profoundly than it does the female. It is quite generally supposed that the action of this one secretion may have much to do with the superior size and vigour of males. For example, Paton says: “The evidence thus seems conclusive that in the male the gonad, by producing an internal secretion, exercises a direct and specific influence upon the whole soma, increasing the activity of growth, moulding the whole course of development, and so modifying the metabolism of nerve and muscle that the whole character of the animal is altered.” It used to be said that the male was more “katabolic,” the female more “anabolic.” These expressions are objectionable, inasmuch as they hint that in a mature organism, with metabolism rather stable, tearing down, or katabolism, could go on faster than building up, or anabolism, or that one of two phases of the same process might go on faster than the other. It seems safer to say merely that a lower metabolism in the female is accompanied by a tendency to store materials.

A long time will doubtless be required to work out the details of differences in metabolism in the two sexes. Some of the main facts are known, however, and the general effects of the two diverse chemical systems upon the life cycles of the sexes are quite obvious. What we call the “quantitative theory of sex” has, besides a place in exact science, an interesting relation to the history of biological thought, especially as applied to society. It is thus in order to state as clearly as possible what it now is; then, so that no one may confuse it with what it is not, to run over some of the old ideas which resemble it.

Experiments with transplanted sex glands, with sex-gland extracts (testicular and ovarian) and the observation of infusions of a male-type blood-stream into a female body, as occurs in nature in some cattle and in the so-called human “hermaphrodites,” indicate a gross chemical difference between the respective determiners for femaleness and for maleness. So the chemicals involved, though not yet isolated, must be presumed to be qualitatively different, since they produce such different results.

But such experiments also indicate that both determiners must be present in some proportions in every individual of either sex. The basis for both sexes being present, the one which shall predominate or be expressed in the individual must depend upon the quantitative relation between the determiners which come together at fertilization. The quantitative theory merely means that this predominance of one factor or the other (maleness or femaleness Gynase or Andrase) is more pronounced in some cases than in others.

In brief, then, the quantitative theory of sex is merely the most reasonable explanation of the known fact that intersexes exist that is, females with some male characteristics, or with all their characters more like the female type than the average, and vice versa. Laboratory biology has established the phenomena of intersexuality beyond question, and the word “inter-sex” has become a scientific term. But the fact that this word and the idea it represents are new to exact science does not mean that it is new in the world.

Intersexes in the human species not only the extreme pathological cases represented by the so-called “hermaphrodites,” but also merely masculine women and effeminate men have been the subject of serious remarks as well as literary gibes from the earliest times. The Greeks called these people urnings. Schopenhauer was interested in the vast ancient literature and philosophy on this subject. The 19th century produced a copious psychological treatment of warped or reversed sexual impulses by such men as Moll, Krafft-Ebing and Havelock Ellis. Otto Weiniger collected a mass of this philosophy, literature, psychology, folklore and gossip, tied it together with such biological facts as were then known (1901) and wove around it a theory of sex attraction. The same material was popularized by Leland, Carpenter and W.L. George to support quite different views.

George’s statement that “there are no men and ... no women; there are only sexual majorities"[, op. cit.] has been widely quoted. The feminists, he adds, “base themselves on Weininger’s theory, according to which the male principle may be found in woman, and the female principle in man.” Unfortunately, George does not make clear what he means by “principle,” so his theory, if he has one, is impossible to appraise in biological terms. From the embryonic idea expressed above, he deduces a very positive social philosophy of sex. The feminists, he says, “recognize no masculine or feminine ‘spheres’ and ... propose to identify absolutely the conditions of the sexes.” So, while George seems to think much more highly of women than does Weininger, their philosophies come together, for quite different reasons, on the practical procedure of disregarding reproduction and letting the race go hang[10, ]. Weininger seems to recognize the dual basis for sex; George evidently does not quite follow him. Both entirely misconceived the real issues involved, as well as the kind of evidence required to settle them, as we shall see later in discussing adaptation and specialization.

Dr Blair Bell[14,15] has collected a mass of evidence on intersexes in the human species. This includes his own surgical and other cases, as well as many treated by his colleagues, and a very considerable review of the medical literature. He not only believes in degrees of femininity in women, but has worked out classifications which he claims to have found of great practical value in surgery.[14, pp.166-7] As noted above, Riddle discovered that his more feminine female pigeons were often killed by a dose of testicular extract which was practically harmless to a partially masculinized female. Sex in the human species being a matter of all the glands organized into a complex, the quantitative “strength” of that complex would be useful to know before removing any one secretion from it. Dr Bell states that the ooephorectomy operation (removal of ovaries) may be performed upon a masculine type of woman with “little disturbance of the metabolism...” But he thinks that the degree of masculinity should always be carefully observed before undertaking such operations, which in some cases have most undesirable effects.

At one end of the scale, this surgeon places the typically feminine woman in all her characteristics with well-formed breasts, menstruating freely and feminine in instincts he says “mind.” The intermediate grades consist, he says, of women whose metabolism leans toward the masculine type. Some have sexual desires but no maternal impulse. Others desire maternity but take no interest in sex activity, or positively shun it. The physical manifestations of masculine glandular activity take the form of pitch of voice, skin texture, shape and weight of bones, etc. Some of the inter-grades are a little hard to define the human species is such an inextricable mixture of races, etc.; but Dr Bell does not hesitate to describe the characteristically masculine woman of the extreme type, who “shuns both sexual relations and maternity...(She) is on the fringe of femininity. These women are usually flat-breasted and plain. Even though they menstruate, their metabolism is often for the most part masculine in character: indications of this are seen in the bones which are heavy, in the skin which is coarse, and in the aggressive character of the mind...If a woman have well-developed genitalia, and secondary characteristics, she usually is normal in her instincts. A feebly menstruating woman with flat breasts and coarse skin cannot be expected to have strong reproductive instincts, since she is largely masculine in type...”

The glandular and quantitative explanation of sex, instead of being abstruse and complicated, brings the subject in line with the known facts about inheritance generally. The dual basis for femaleness and maleness in each individual simply means that both factors are present, but that only one expresses itself fully. The presence of such a dual basis is proved by the fact that in castration and transplantation experiments both are exhibited by the same individual in a single lifetime. In the case of the Free-Martin cattle, even the female sex-glands are modified toward the male type to such an extent that they were long mistaken for testes. The same applies to some glands found in human “hermaphrodites,” as Dr Bell’s plates show.

The peculiar complication of the chemical complex determining sex in these mammalian forms, involving all the glands and hence the entire body, makes it problematical whether a complete (functional) reversal is possible, at least after any development whatever of the embryo has taken place. On the other hand, the fact that such complete transformations have not so far been observed by no means proves their non-existence. Their being functional, and hence to all external appearances normal, would cause such animals to escape observation.

Latent traits of the opposite sex of course immediately suggest recessive or unexpressed characters in the well-known Mendelian inheritance phenomena. In the bird-castration cases, we saw that to remove the inhibiting sex glands caused previously latent characters to act like dominant or expressed ones. The case of horns in sheep, investigated by Professor Wood, is so similar that it seems worth summarizing, by way of illustration.

Both sexes in Dorset sheep have well-developed horns; in the Suffolk breed both sexes are hornless. If the breeds are crossed, all the rams in the first (hybrid) generation have horns and all the ewes are hornless. If these hybrids are mated, the resulting male offspring averages three horned to one hornless; but the females are the reverse of this ratio one horned to three hornless. This is an example of Mendel’s principle of segregation factors may be mixed in breeding, but they do not lose their identity, and hence tend to be sorted out or segregated again in succeeding generations.

In the horned Dorsets, we must suppose that both males and females carry a dual factor for horns technically, are homozygous for horns. The hornless Suffolks, on the contrary, are homozygous for absence of horns. Thus the dual factor in the zygotes or fertilized eggs at the basis of the first filial (hybrid) generation consists of a single factor for horns and a single factor for their absence. If we represent horns by H and absence of horns by A, Dorsets have a factor HH, Suffolks AA and the hybrids HA.

All the males in this generation have horns, which means that a single “dose” of the factor H will produce horns in a male, or that they are dominant in males. But a single dose will not produce horns in a female that is, horns are recessive in females the factor is present but unexpressed.

Mating two HA hybrids, the H and A of course split apart in the formation of the gametes, as the HH and AA did in the previous generation; so that we get an equal number of single H and A factors. In reuniting in fertilized eggs, the chance is just half and half that an H will unite with another H or with an A that an A will unite with an H or another A. Thus we have two chances of getting HA to each chance of getting either AA or HH. Half the zygotes will be HA, one-fourth HH and one-fourth AA.

If we consider four average males, one will have two A’s (absence of the factor for horns) and will thus be hornless. One will have two H’s, or the double factor for horns, and hence will exhibit horns as will also the two HA’s since a single dose of horns expresses them in a male. So we have the three-to-one Mendelian ratio.

But four females with exactly the same factors will express them as follows: The one HH (double factor for horns) proves sufficient to express horns, even in a female. The AA, lacking the factor entirely, cannot have horns. Nor will the two HA females have horns, a single dose being insufficient to express them in a female. Again we get our three-to-one Mendelian ratio, but this time it is three hornless to one horned.

Especially Goldschmidt’s carefully graded experiments point to a similar difference in the strength of the dose or doses of the sex factors. Instead of the two doses of horns required to express them in the presence of the female secretory balance in Professor Wood’s sheep, Goldschmidt found it took six doses of maleness to completely express it on a female basis in his moths. But even with three doses, the female was incapable of reproduction. A single dose in excess of the ordinary combination to produce normal females modified the type of body, also reducing the number of eggs.

In the case of the horns, only two types were possible, absence or presence of the character. Likewise there are only two types of primary sex, i.e., of sex glands proper. But seven different types or grades of body for each sex were found to exhibit themselves in the moths. In more complicated bodies, we should of course expect many more, and where many races (instead of two) are mixed, as in man, a classification merely on the basis of physical characteristics would be much more complicated. Indeed, we may well be sceptical as to the possibilities of cataloguing differences of the sort between men and women by body type alone.

In society, however, we are much more interested in the mental than the purely physical qualities of the two types of bodies, especially since the use of machines has so largely replaced brute strength with skill. Most employments do not even require a muscular skill beyond that possessed by ordinary individuals of both sexes.

Even this ignores the primary consideration in the sex problem in society, the first of the following two parts into which the whole problem may be divided: (1) How to guarantee the survival of the group through reproduction of a sufficient number of capable individuals; and (2) How to make the most economical use of the remaining energies, first in winning nutrition and protection from the environment, second in pursuing the distinctly human values over and above survival. The sex problem as a whole is concerned with adjusting two different general types of individuals, male and female, to the complicated business of such group life or society. The differences between these two sex-types being fundamentally functional, the best way to get at them is to trace the respective and unlike life cycles.

We have already shown in rude outline how a difference (apparently chemical) between two fertilized eggs starts them along two different lines of development in the embryonic stage. One develops the characteristic male primary and secondary sex characters, the other the female. Throughout the embryonic or intra-maternal stage this differentiation goes on, becoming more and more fixed as it expresses itself in physical structures. Childhood is only a continuation of this development physically separate from the mother after the period of lactation. Until puberty, when sex ceases to be merely potential and becomes functional (about 12-14 in girls and 14-16 boys), the differences in metabolism are not very marked. Neither are they in old age, after sex has ceased to be functional. It is during the period when sex is functional (about 35 years in women and considerably longer in men) that the gross physiological differences manifest themselves.

Before puberty in both sexes, calcium or lime salts are retained in the tissues and go to build up the bony skeleton. (A mere sketch of calcium metabolism is all that can be given here for details consult such works as 15 and 17 in bibliography; summary in 14; pp. 34f. & 161f.) Note that puberty comes earlier in girls than in boys, and that the skeleton therefore remains lighter. During the reproductive period in women these salts are heavily drawn upon for the use of the reproductive system. The male reproductive system draws upon them as well, though the drain is very slight as compared to that in women. In old age these salts produce senility through deposit in the tissues, especially in the arteries.

At the pubertal age in girls begins the phenomenon known as menstruation, in which there is a large excretion of calcium salts. In pregnancy these are needed for building the skeleton of the foetus, and at delivery go to the breasts to assist in lactation. Bell states that there is a noticeable connection between early menstruation and short stature, and vice versa. What is commonly known as menstruation lasts only a few days, and is merely the critical period in a monthly cycle or periodicity which goes with the female sex specialization. This period involves the gradual preparation of the uterus or womb for a guest, together with the maturing of the ova. Then the Graafian follicles containing the ova break and these latter enter the uterus for fertilization.

If fertilization takes place, the fertilized egg buries itself in the wall and development of the embryo proceeds. Menstruation stops, the calcium salts being required for the growing embryo. There is likely to be no menstruation for a considerable time after delivery if the child is nursed, as is normal. This gives the uterus time for devolution to the normal, before a surplus of calcium salts sets the periodicity going again. If the egg which passes from the ovary to the uterus is not fertilized, it is excreted, the uterus goes through another monthly cycle of preparation for the period of intra-maternal environment, and so on indefinitely until the climacteric.

This climacteric or decay of sexuality is a rather critical time, especially in women. It marks the period at which the metabolism can no longer support the strain of reproduction. A surplus of calcium brings on senility, as noted above. Withdrawal of the interests which centre in sex, together with the marked accompanying physical changes, involves a shift of mental attitude which is also frequently serious. A British coroner stated in the British Medical Journal in 1900 (Vo, that a majority of 200 cases of female suicide occurred at this period, while in the case of younger women suicide is peculiarly likely to occur during menstruation. Krugelstein and Lombroso, respectively, remark the same tendencies.

It is a matter of almost everyday observation that men and women in the neighbourhood of fifty suddenly find themselves disoriented in the world. Tolstoi, for example, who had written passionately of passion in his earlier years, suddenly awoke, according to his “Confessions,” from what seemed to him afterward to have been a bad dream. In this case, the result was a new version of religion as a new anchorage for the man’s life. It may be pacifism, prohibition, philanthropy, or any one of a very large number of different interests but there must usually be something to furnish zest to a life which has ceased to be a sufficient excuse for itself.

If freed from worry about economic realities, it is not infrequently possible for the first time for these people to “balance” their lives to find in abstraction a rounded perfection for which earlier in life we seek in vain as strugglers in a world of change. Thus old people are often highly conservative, i.e., impatient of change in their social environment, involving re-orientation; they wish the rules of the game let alone, so they can pursue the new realities they have created for themselves.

Socially, the old are of course a very important factor since a changed metabolism sets them somewhat outside the passionate interests which drive people forward, often in wrong directions, in the prime of life. Hence in a sense the old can judge calmly, as outsiders. Like youth before it has yet come in contact with complicated reality, they often see men and women as “each chasing his separate phantom.”

While such conservatism, in so far as it is judicial, is of value to society, looking at it from the viewpoint of biology we see also some bad features. Senex, the old man, often says to younger people, “These things you pursue are valueless I too have sought them, later abandoned the search and now see my folly;” not realizing that if his blood were to resume its former chemical character he would return to the quest.

Elderly people, then, biological neuters, come especially within the problem of the economical use of the social as distinguished from the biological capacities of the race. They affect the sex problem proper, which applies to a younger age-class, only through their opinions. Some of these opinions are hangovers from the time in their own lives when they had stronger sexual interests, and some are peculiar to people of their readjusted glandular activity. Their reproductive contribution to society has been made.

Pre-pubertal childhood and youth, on the contrary, has its biological contributions to society still before it. The glandular activity of boys and girls is perhaps not so unlike as to justify society in giving them a different kind of education and preparation for group life. The excuse for two sorts of training is that the two sexes will not do the same work after puberty. Hence the question of youthful training is sociological almost entirely not biological or rather, it rests upon the biology, not of childhood but of the reproductive period, which society anticipates.

Instead of scattering attention over the whole history of the universe, then, or even over the general field of biology, in dealing with sex as a social problem, the emphasis must be upon the human life cycle during the functional-reproductive period. Other biological data than that which concerns this period is merely introductory or explanatory. The extent to which the sociological problem involved is linked up with general biological considerations like natural selection, adaptation and specialization will be summarized in a separate chapter.

Earlier female maturity and puberty, as well as lighter structure, have already been accounted for by the metabolism, especially of the calcium salts. These have also been shown to be the key fact in the monthly periodicity of the mammalian female. Nearly all of the anatomical and physiological sex differences catalogued by such pioneer workers as Ellis, Ploss, Thomas and Bucura are simply what we should expect from the less active and in some ways peculiar metabolism of woman.

Among such differences are the size and shape of bones and other body structures, the more plentiful haemoglobin in male blood during the reproductive period, and such blood peculiarities as the production of more carbonic acid or the higher specific gravity in the male. The greater percentage of fat as compared with muscle in women, if it is generally true, is what we should expect from a lower metabolism and a tendency to store materials. The long list of diseases which are more or less sex-limited [20; 14, pp.160f.; 18] are largely endocrine. Even those which do not primarily concern the internal secretory system would be expected to work somewhat differently in the presence of unlike blood streams. As to the greater average weight of the male brain, this is true of the whole body. Brain weight, either absolute or relative to body weight, is not positively known to be in any way correlated (in normal people) with mental capacity.

A library might be stocked with the vast literature devoted to summarizing and cataloguing sex differences; and most of it would be useless from the standpoint of sociology. Unaccompanied by the criticisms a biologist would have to make on the method of their ascertainment and validity, not to mention their significance, such lists can easily do and probably have done more harm than good. One simple and reasonable criterion would reduce this catalogue to fairly modest proportions, so far as social science is concerned: Which ones have an obvious or even probable social significance? Over and above that, while such contrasts may be of speculative interest, they lead imaginative people to argue from them by analogy and thus cloud the real issues.

What are the outstandingly significant sex differences which application of the above criterion leaves? (1) A less active and more uneven metabolism of woman; (2) Associated with this, less physical strength on the average hence an inferior adaptability to some kinds of work, resulting in a narrower range of choice of occupation, disadvantageous in competitive society; (3) But the one fundamental difference, to which all the others are as nothing, is the specialization of the mammalian female body and metabolism to furnish the intra-maternal environment (approximately nine months in the human species) for the early development of the young and lactation for some months afterward.

This last may be said to include the former two, which were arbitrarily placed first because they are always in evidence, whether reproduction is undertaken or not. This takes us out of cell and endocrine biology and into the general problem in group adjustment to environment which that specialization entails.